U.S. Pat. No. 2,350,440 to John O. Almen discloses a device used for measuring the intensity of shot blast peening on metal parts, commonly known as an Almen gage. Almen gages measure the intensity of the peening process by measuring the curvature of thin metal test strips, after one side of the test strips have been peened. The unbalanced stress in the peened surface causes the test strips to bow. The Almen gage uses two spaced knife edged supports and a measurement indicator with an reciprocating feeler to measure the curvature of the test strip. The spaced knife edged supports hold the test strip as the feeler located between the knife edges engages the test strip. Gaging the height of the arc or bend of the test strip between the two predetermined contact points along the knife edged supports provides an indication of the intensity of the peening process.
Later refinements of the Almen gage have replaced the knife edged supports with four round contact balls with the operator manually holding the test strip in place for measurement. Another version of the Almen gage added a spring loaded finger to hold the test strip in place. The spring loaded finger was difficult to operate and subject to abuse and damage. The latest revision in the Almen gage incorporated a large permanent magnet centrally mounted in the base to draw the strip against the contact balls. Generally, the magnet was spaced some distance from the test strip, which necessitated the use of a large powerful magnet. The magnetic flux of such large magnets tended to deflect the thin test strips and corrupt the measurement readings. The location of the magnet also forced the indicator stems to be constructed of non-magnetic materials in order to avoid magnetic influence, which would affect the indicator's free travel.